This invention relates generally to P/N junction devices such as semiconductor diodes and thyristors, and more particularly the invention relates to such junction devices having improved turn off characteristics.
In forward biased P/N junction semiconductor devices such as PIN diodes and PNIPN thyristors, a large amount of charge can be stored in the highly injected intrinsic standoff region during conduction (on state) of the device. The stored charge must be removed during turn off of the device. This can be accomplished either through carrier recombination or by transport of the carriers to the anode and cathode contacts. The conventional method of achieving fast turn off is to create mini defects in the semiconductor body which act as recombination centers. This is usually done either by diffusing gold into the device or by creating crystal defects by electron irradiation. Both of these techniques have draw backs; gold diffusion can create precipitates which reduce breakdown, and electron irradiation crystal damage can be annealed out which limits permissible die attach and soldering temperature.
Speed enhancement for lifetime reduction is limited by more fundamental issues, however. In order to maintain sufficient conductivity modulation, it is necessary that the ambipolar diffusion length, L.sub.a, be longer than half the stand off region thickness, W. This requires that: ##EQU1## where .tau..sub.hl is the high-level lifetime and D.sub.a is the ambipolar diffusion constant. This gives .tau..sub.hl .apprxeq.1 .mu.S when W=100 .mu.m. The switching time will be roughly equal to this. As the lifetime is decreased from this value, the on-voltage will increase rapidly in an exponential manner.
Because of the exponential dependence of carrier density on distance when recombination is present, charge control by lifetime reduction results in excess charge at the ends of the stand-off region while starving the center. The voltage drop across the stand-off region is either quite small (less than 0.2 V or so) or very large as the diffusion length becomes too short. The result is that more charge is stored than is really needed.